1. Field of the Invention
This invention relates to lock mechanisms for doors. More particularly, this invention relates to lock mechanisms which utilize cylindrical bored lock sets.
2. Description of the Prior Art
In a typical cylindrical or bored lock system (shown in drawing FIG. 1) the lock cylinder is generally enclosed and protected by the outer grip knob of the lock set. The outer and inner grip knobs are coaxially mounted to spindles which are connected to a mechanism that resists knob rotation when the lock controls are set in the locked position. The spindles interact with a latch retractor, and rotation of the spindles when the lock is unlocked will impart transverse motion to the lock retractor, which in turn retracts the latch bolt of the assembly.
The conventional cylindrical lock mechanism has several disadvantages which limit its usefulness in areas having high security requirements. Initially, since the lock cylinder is contained within the outer doorknob, it is subject to easy manipulation. A vandal simply has to remove the doorknob in order to expose the latch retracting mechanism. This mechanism may then be easily manipulated with a common tool such as a screwdriver. Manipulation of the latch retracting mechanism enables the latch bolt to be retracted. In addition, removal of the lock cylinder and cylinder spindle permits direct manipulation of the latch retractor and withdrawal of the latch bolt.
An additional problem with lock mechanisms of the above described type is that it is usually possible to apply sufficient torque through the outside grip knob to cause failure of a lock tab which prevents rotation of the outside knob when the lock controls are set in the locked position. Upon failure of the lock tab, the outside grip knob may be rotated until retraction of the latch bolt has been accomplished.
Because of the above described security problems with cylindrical bored locks, most high security installations utilize lock mechanisms which incorporate some type of deadbolt arrangement in addition to the normal latch bolt. Although these types of locks provide high security, they are also complex and therefore usually relatively expensive.
One deadbolt arrangement is disclosed in U.S. Pat. No. 3,990,277 assigned to the same assignee as the present invention. This mechanism incorporates an exterior knob which is offset from the interior knob. Such an arrangement permits a mechanism to be used which decouples the exterior knob from the interior knob when the lock is engaged.
In a cylindrical bored lock, a deadbolt is often not employed and it becomes very important to protect the lock cylinder from tampering. It is therefore a primary object of the present invention to provide a cylindrical bored lock assembly which guards the lock cylinder and resists tampering.
It is another object of the present invention to achieve a cylindrical bored lockset which approaches deadbolt systems in terms of security but is much simpler and less expensive than such systems.